1. Field of the Invention
This invention relates to a viewfinder mounted on a camera separately from a taking lens, and more specifically to a viewfinder having a prism for correcting a parallax relative to the taking lens upon a macro photographing mode.
2. Description of the Prior Art
Heretofore there has been a camera including a viewfinder separated from a taking lens such as a lens shutter camera or a still video camera. The viewfinders of these conventional cameras have in principle parallax by the difference in the direction of optical axis from that of the taking lens. This parallax can vary according to the distance from a camera to a photographic object, and is needed to decrease in actual photographing.
Besides a normal photographing mode, some of the conventional cameras have a macro photographing mode for taking a short-distance object which can not be taken by the normal photographic mode.
Since a normal mode camera is preset to minimize the parallax for normal photographing, it has high quantity of parallax in the short distance for macro photographing.
In order to minimize this parallax upon macro photographing, some of the conventional cameras have a rotatable correcting prism by standing across the optical path of the viewfinder and other cameras have a movable visual field frame in the viewfinder optical system.
FIG. 12 shows a schematic view for inserting a prism. This reversed Galilean zoom viewfinder has a first lens group 41, a second lens group 42 movable for variable power, and an eyepiece group 43. The magnification of the viewfinder is changeable according to that of a taking lens. The parallax is decreased by inserting the prism 44 between the first lens group 41 and the second lens group 42 and thereby bends its optical path to the taking lens side upon macro photographing.
FIG. 13 is a schematic view showing the moving of the visual field frame of a viewfinder. This zoom viewfinder has a first lens group 51, a second lens group 52 movable for variable power, a Porro prism for erecting images, and an eyepiece 54. A visual field frame 55 is mounted in the vicinity of the image formation position of a photographic object. The parallax upon macro photographing is corrected by moving the visual field frame in the direction normal to the optical path and thereby an accurate visual field is gained.
Referring to FIG. 12, the prism on the optical axis is rotated in order to insert into the optical path and to extract from the path, and requires a comparatively wide space for its rotation at the same time. Therefore the lens-arrangement is restricted by this space.
In FIGS. 12 and 13, a high magnification of the viewfinder causes a great difference between the diopter (i.e., the magnification power of the viewfinder) upon macro photographing to a short distance object, and that upon normal photographing to a standard distance object. A range of the above difference often surpasses the adjustable range of the diopter of a photographer's eye. Since a standard diopter is preset generally within the available distance for normal photographing, a large change of the diopter for macro photographing from a standard diopter makes fuzziness of an image in the viewfinder upon macro photographing.
When the distance to a photographic object is defined as U, measured in meters, the standard object distance for setting a diopter is defined as U.sub.o measured in meters, and the magnification of the visual field in the viewfinder is defined as M, the diopter difference between one object located at U.sub.0 and another object located at U will be as follows: EQU .DELTA.D=(U.sup.-1 -U.sub.0.sup.-1)M.sup.2
For example, when M=1.2, U.sub.0 =3 m, and the standard diopter D=-1, are each defined, the result is .DELTA.D=0.96, and D=-1.96 in the macro mode in the case of U=1.
Both a high magnification of the viewfinder and a large difference between the standard distance for adjustable diopter and the actual distance requires much change from the standard diopter. Recently, zoom ratio, in another word, a difference between the focal distances of a short focal distance side and a long focal distance side, intends to become large. Therefore, the diopter upon macro photographing by long focal distance to a short distance object largely changes from standard diopter for normal photographing.